The present invention relates to a lockup torque converter generally used in an automatic transmission.
A lockup torque converter has two operating modes. In one operating mode (torque conversion state), a vortex flow of oil impelled by an engine-driven torque converter input member (which is usually a pump impeller) drives an output member (which is usually a turbine runner) while multiplying torque under the reaction of a stator. In the other operating mode (lockup state), a lockup clutch is engaged to provide a direct mechanical driving connection between the input and output members. A lockup torque converter uses the torque conversion mode in a relatively low engine speed range in which engine torque fluctuation has a harmful influence, and there is a demand for multiplying torque. In a high engine speed range (a high vehicle speed range), a lockup torque converter uses the lockup mode and thereby eliminates slip between the input and output members. In this way, a lockup torque converter can improve fuel economy, as compared with a conventional torque converter which does not have the lockup mode and therefore suffers slippage at high speeds.
A lockup torque converter usually chooses one from these two operating modes depending upon whether the vehicle speed is higher than a predetermined vehicle speed called a lockup vehicle speed. This lockup vehicle speed is set at such a high vehicle speed that engine torque fluctuation becomes so low as not to cause vehicle body vibration. Therefore, a lockup torque converter having only these two operating modes can use the lockup mode only in a narrow range above such a high lockup speed, and its fuel economy advantage is not sufficient.